The remediation of soil and groundwater contaminated by light (lighter than water) non-aqueous phase liquids (LNAPL) and dense (heavier than water) non-aqueous phase liquids (DNAPL), collectively known as non-aqueous phase liquids (NAPL), remains a difficult problem where these contaminants exist as a residual (undissolved) or free product within a soil and/or rock matrix. LNAPLs (e.g., oily substances that float in water) and DNAPLs (e.g., chlorinated solvents, coal tar, creosote, that sink in water) are not readily removable in their entirety and continue to contaminate groundwater and soil and prevent or restrict use of the site in which these contaminants exist, or pose a threat of migration onto nearby properties. In the case of LNAPL, free product can adversely impact soil and groundwater and migrate onto other properties. Sometimes the LNAPL contains in solution other compounds that are even more toxic than the pure LNAPL (e.g., polychlorinated biphenyls [PCBs] or benzene), and in this case a less toxic LNAPL can serve as a transport medium for the more toxic compound(s).
One of the difficulties in remediation of LNAPLs and DNAPLs is that the mass or volume of contaminant is poorly understood or completely unknown. In the case of LNAPLs, invalid methods are most often employed to estimate the volume of contaminant, so the recoverable quantity of contaminant is overestimated or underestimated and the benchmarks for cleanup are uncertain or unknown; largely they are unattainable and unrealistic using conventional products and methods. The non-recoverable LNAPL, that portion sequestered within the soil or rock matrix, i.e. the residual, remains behind in pore spaces or voids and continues to adversely affect soils and groundwater. The residual is vastly more difficult to remove than the free product using conventional in situ methods and, in practice, removal is effectively infeasible.
In the case of DNAPLs, the contaminant volume and/or mass is also uncertain or unknown. Again, accurate, reliable estimates of the contaminant mass frequently do not exist. Removal of subsurface DNAPL is very difficult because these compounds dispense, forming fingers and pools making them very hard to locate and accurately quantify. Most of the DNAPL exists as a residual that occupies the pore spaces or voids and is exceedingly difficult to remove, treat, or otherwise access, depending on the site geological characteristics. As with LNAPL, DNAPL residual removal is effectively infeasible.
Removal of LNAPL and DNAPL meets with varying degrees of success depending on the recovery or treatment method, the understanding of the contaminant, geology, the mass or volume present and available for removal or treatment, and clear and attainable benchmarks for cleanup.
The reason why LNAPLs and DNAPLs are so difficult to remove is that the contaminants occupying the small voids, pore spaces, or apertures within the soil or rock matrix are strongly held and effectively immobilized by capillary forces. Depending on the size of the pores, voids, or apertures, the LNAPL or DNAPL is more strongly or weakly held; smaller openings hold contaminants more strongly No known conventional technology can effectively remove contaminants from the pores or voids while the geologic media remains in place (in situ).
Most often, contaminant levels are compared against numerical cleanup standards for soil or groundwater. However, the remedial process frequently, for practical purposes, ignores the mass or volume of contaminant. Effective remediation of the source mass or volume is paramount if remediation is to ultimately be effective and restore groundwater resources, soil, and real estate to productive use and protect the public and the environment. Removal and/or destruction of contaminant mass are of overriding importance.
One example of ground contamination remediation is discussed in U.S. Pat. No. 4,435,292. In this method, perforated pipes and wells are inserted into the ground of a contaminated site, wherein a number of the pipes and wells are pressurized and others are simultaneously evacuated to effect the transfer of flushing fluid through the soil to accelerate removal of contaminants, and to prevent migration of contaminants into other areas. The system is closed and pressurized at one end and evacuated at another end, for example, by evacuating ducts connected to a central pressure manifold. The flushing fluid may be either liquid or gaseous, e.g. an inert gas such as nitrogen, or a reactive system which would react with the contamination to form an inert or harmless chemical.
The process, however, suffers from the need to have a reliable benchmark as to the mass or volume of contaminant present so as to know how much treatment chemical is required and for how long treatment will take, and a reliable benchmark as to when the contaminant has been effectively neutralized or destroyed. The process relies on subsequent soil and groundwater contaminant measurements to determine when treatment is complete. These types of measurements are notoriously variable and a great many data points from a plurality of locations, over time and in several seasons are required to evaluate whether treatment is complete. Another important limitation of this approach is that the greatest contaminant concentrations do not necessarily coincide with the location of the greatest amount of contaminant mass or volume. Even with abundant measurements, rebound, i.e. the contamination from residual contaminant that continues to migrate back into groundwater, may appear well after the data suggest that remediation is complete. Without a reliable before and after estimation of volume or mass, effective treatment is uncertain and questionable. This issue is of great importance to environmental regulatory agencies, or other bodies charged with deeming remediation complete to protect the public and the environment.
The flushing process is also dependent on the geologic media that control fluid movement and how effectively the treatment method reaches the contaminants. Most commonly, treatment fluids follow preferential pathways, also known in the field as “fingering,” channels of easiest fluid movement and, as such, treatments and/or removal processes reach only a small percentage of the contaminant mass; most of the contaminant mass remains untreated, where it continues to adversely impact soil and groundwater. This method (U.S. Pat. No. 4,435,292) does not have the capacity to alleviate fingering as it relies on the inherent geologic properties and does not alter, i.e. increase, the conductivity of the geologic medium so as to promote or enhance remediation.
Another attempt at soil and groundwater decontamination is described in U.S. Pat. No. 5,279,740. This process represents an improvement over the aforementioned approach and consists of a mechanism of contaminant removal using at least two injection wells positioned in the contaminated zone and at least one extraction well to remove the mobilized contamination. Steam is then introduced into the ground and forced into the contaminated zone while simultaneously introducing treatment agents, if desired. A removal force is then applied to the extraction well for withdrawal of the contaminants. Enhanced removal and treatment are contemplated using this process. In an ideal setting, an array of steam injection wells and extraction wells covers the contaminated area. This process suffers from the same limitations noted in the first example. Without reliable estimates of contaminant mass or volume, the same deficiencies remain with regard to lack of meaningful benchmarks to gauge before and after treatment. The second example contemplates the use of an extraction well and an extraction force, but the approach is subject to the same limitation caused by preferential pathways, “fingering” that causes contaminant removal or treatment to contact only a fraction of the total contaminant mass, and typically the mass that is most easily treated and/or removed. Again, this process does not alter the conductivity of the geologic medium so as to promote or enhance remediation.
Other methods to alleviate soil and/or groundwater contamination employ the creation of a vacuum within a withdrawal well situated in the vadose zone. Air injected into the well at various points surrounding the withdrawal well urge the flow of contaminants towards the withdrawal well where they are vaporized and collected in the well by vacuum. Examples of this method are described in U.S. Pat. Nos. 4,593,760 and Re. 33,102.
A variation of the vacuum method mentioned above is discussed in U.S. Pat. No. 4,730,672, which presents a method for removing and collecting volatile liquid contaminants from a vadose zone of contaminated ground by an active closed-loop process, in which a vacuum source in a perforated conduit in a withdrawal well is situated in a contaminated vadose zone and creates a reduced pressure zone to cause contaminants contained therein to vaporize and be drawn in to the withdrawal conduit for collection and disposal. While effective for the removal of some easily volatilized liquid contaminants in the vadose layer, such methods have proved to be of limited value in the removal and disposal of many other common subsurface contaminants. Additionally, such methods are not useful for removal of contaminants situated below the water table in a saturated zone.
All the methods described above are employed either with or without any reliable measure of contaminant mass or volume, before and/or after, and work within the existing geologic framework. The effectiveness of these measures is dictated or limited by the existing porosity, voids, or aperture size, and permeability of the geologic media within which the contamination resides. One characteristic all the aforementioned methods have in common is that they treat the geologic conditions as though they are static and immutable. They focus exclusively on the concentration levels of contaminant and neglect soil, geologic and fluid physical properties.
Accordingly, there is a need for an integrated assessment-remediation process that accurately estimates the volume or mass of NAPL, rapidly removes the NAPL and/or treats the NAPL in situ, and then quantitatively evaluates whether the contaminant volume or mass has been remediated following treatment. There is a need for an evaluation process that factors both soil and fluid properties and/or changes in contaminant mass in estimating their mass and/or volume before, after and/or during treatment so as to monitor/adjust treatment effectiveness in real time. There is a need to more efficiently access the contamination so that it may be rapidly physically removed from the geologic media and/or treated with an agent that destroys and/or neutralizes the contaminant or otherwise renders it non-toxic. Restoration by means of rapidly altering the physical properties of the geologic media, e.g. porosity, conductivity and permeability, in the saturated zone below the water table and/or the capillary fringe above the water table, is fundamental to the process. By promoting more effective in situ remediation, the public will be protected because it will not be exposed to excavated contaminant that frequently results in noxious odors and toxic or nuisance particulates.